The present invention concerns a method of determining a partitioning of a digital image for inserting a watermarking signal, a method of inserting a watermark, a method of processing an image and an associated method of decoding a watermarking signal.
Correlatively, it concerns a device for determining a partitioning of a digital image for inserting a watermarking signal, a device for inserting a watermark, a device for processing an image and a device for decoding a watermarking signal.
The present invention lies in general terms in the technical field of watermarking of digital images, and more particularly still images.
Watermarking digital data makes it possible to protect these data, for example by associating copyright information with them.
In its general principe, watermarking consists of inserting an indelible watermark in digital data, which can be likened to encoding additional information in the data.
Decoding this additional information makes it possible to check the additional information which has been inserted.
This inserted watermark must consequently be both imperceptible, robust to certain distortions applied to the digital image and reliable to detect.
Conventionally, a usual technique of inserting a watermarking signal in a digital image consists of using a linear modulation model in which at least one subset of coefficients representing the digital image is modulated according to this linear model using a weighting coefficient.
By denoting as X={Xi, 1≦i≦N} a set of the coefficients representative at least of part of a digital image, and as w={wj, 1≦j≦P} a watermark of size P≦N, a pseudo-random signal with known distribution and average of zero, the linear insertion formula is:X′j=Xj+b αjwj, with 1≦j≦P,
in which {Xj, 1≦j≦P} is a subset of the set of coefficients X, b is an information bit, and αj is a weighting coefficient, also called modulation amplitude.
The detection of the watermark then consists in detecting whether or not the pseudo-random sequence w has been inserted into a set of coefficients. This detection is carried out without using the original image and can be based on a standardised statistical test which makes it possible to calculate a probability of correct detection.
Such an insertion technique makes it possible, by insertion of a watermark, to insert a single information bit since the response from the detector is binary (yes/no).
In order to insert a larger number of information bits into the digital image, in particular when a code of Q bits is wanted, indicating, for example, the name or the address of the owner or of the author of the image, it is necessary to reiterate the previously described insertion method as many times as there are information bits to be inserted. Typically, in order to insert a binary signal, either b=1, or b=−1 is used.
Put another way, Q subsets of coefficients have to be chosen and the modulation of these subsets has to be carried out by choosing Q watermarks.
Separate subsets of coefficients are preferably chosen such that the modulation operations are not superimposed on one another, which could disturb the detection or cause troublesome visual effects.
It is a matter, consequently, of choosing a partition of the coefficients representative of the digital image into Q separate subsets, each carrying one information bit.
Numerous known methods use a technique of inserting a watermarking signal of given size by spectrum spreading. The disadvantage of these methods, describe for example in the article entitled “Secure spread spectrum watermarking for multimedia” by I. J. COX et al, in Proc. ICIP, pages 243–246, September 1996 and in the article entitled “Digital watermarking of raw and compressed video” by F. HARTUNG et al, in Proc. SPIE 2952: Digital Compression Technologies and Systems for Video Communication, pages 205–213, October 1996, is that they use an arbitrary partitioning of the image into blocks of fixed size without any guarantee of detectability of the modulations effected on each of the blocks.